Linear dichroic properties of polyethylene films containing a dispersion of terthiophene-based chromophore molecules are investigated by using a scanning near-field optical microscope (SNOM). The polarization-modulation technique implemented in our SNOM provides quantitative information on the dichroic ratio of the samples with sub-wavelength space resolution. Optically active domains are identified and their morphology is analysed as a function of the film fabrication parameters, e.g., the drawing ratio and the kind of dispersed chromophore mixture. These investigations complement conventional polarimetry analysis by adding nanometre-scale information on the spatial distribution of the chromophore molecules and their mutual alignment with the host polymer chains.